A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by an auditory prosthesis system such as a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple stimulation contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format, such as a sequence of data frames, for transmission into a receiver processor in an implant housing 108. Besides extracting the audio information, the receiver processor in the implant housing 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110 which penetrates into the cochlea 104 through a surgical opening in the outer surface of the cochlea 104. Typically, this electrode array 110 includes multiple stimulation contacts 112 on its surface that deliver the stimulation signals to adjacent neural tissue of the cochlea 104 which the brain of the patient interprets as sound. The individual stimulation contacts 112 may be activated sequentially, or simultaneously in one or more contact groups.
To implant the electrode array 110 into the cochlea 104, a posterior tympanotomy is performed at the facial recess opening in the mastoid process of the temporal bone. This involves creating an opening through the mastoid air cells to obtain surgical access from the implant housing 108 which is affixed on the outer surface of the temporal bone beneath the skin behind the pinna of the outer ear 101, to the air space of the middle ear 103. FIG. 2 is a photograph of a facial recess 200 showing the oval shape of the posterior tympanotomy opening that is created, which has a long diameter 201 and a short diameter 202. The electrode array 110 can then be routed from the implant housing 108 through an opening in the outer surface of the cochlea 104.
FIG. 3A shows structural details of a cochlear implant electrode arrangement at the electrode opening 301 into the implanted cochlea 104. After the insertion procedure, the electrode array 110 tends to lie toward the outer lateral wall of the spiral-shaped cochlea 104. Over time, there can be a tendency for the electrode array to spring back and retract back out through the electrode opening 301, as shown in FIG. 3B. The degree of electrode spring back varies depending on how deeply the electrode array 110 is inserted into the cochlea 104, how well the electrode opening 301 is packed with fascia material, and the specific geometry at the electrode opening 301.
Such post-surgical electrode retraction pulls the nearest stimulation contact 112 away from its intended target neural tissue within the cochlea 104 back toward the electrode opening 301, or even further, back outside the cochlea 104 into the middle ear 103. This can produce pain sensation in the patient when that stimulation contact 112 is energized. Usually in such circumstances, that stimulation contact 112 will be inactivated and fewer stimulation contacts 112 remain for use to produce sound sensation. In some cases, an additional revision surgery may even be needed to push the electrode array 110 back inside the cochlea 104.
Various approaches have been attempted to resist such post-surgical retraction by implementing various anti-retraction structures at the electrode opening in the outer surface of the cochlea. A cork-shaped stopper has been used to tightly wedge the electrode lead in the electrode opening. An anti-retraction skirt has been implemented on the electrode array at the electrode opening which is made of polymer material that swells when contacted by the liquid preilymph medium, thereby holding the electrode array in place. Some electrode arrays have a permanent pre-curved shape that does not relax or spring back after insertion into the cochlea. Other electrode arrangements contain an internal malleable material on either side of the electrode opening which maintains a bent shape after full insertion of the electrode array to resist retraction. A surgical group in Hannover Germany has added to the implant electrode a wing of flexible silicone material that can be fixed to a groove in the bony material on the outer surface of the cochlea near the electrode opening. All of these efforts have suffered from various issues that leave each an imperfect solution.